Portable computers and various other portable electronic devices include a battery that must be periodically re-charged. Separate power supplies that are used to recharge batteries are very common devices. Frequently external power supplies connect to electronic devices that include batteries by means of a power cable. Alternatively, many electronic devices can be plugged into docking stations that include a fixed connection to an external power supply.
Many electronic devices include Universal Serial Bus (USB) ports. The USB ports on currently available electronic devices generally conform to either the “Universal Serial Bus Revision 1.1” specifications” or to the “Universal Serial Bus Revision 2.0” specifications. These specifications are widely known and widely available.
Ports that conform to the USB specifications can supply power from the host to peripheral devices. That is, a USB bus connection can be used to supply power to a peripheral device. The USB specification defines a low power mode and a high power mode. In the lower power mode the host provides no more than 100 ma of current at five volts. In high power mode, the host can provide up to 500 ma of current at five volts. Thus, according to the USB specification a maximum of 2.5 watts of power can be provided from the host to a peripheral device on a USB port.
In all cases, the USB specifications are directed to providing power from the host to the peripheral devices. The power is provided via the VBUS and GND lines that are two of the four lines in standard USB cables and connectors.